Re: fstrim is takes a long time on Btrfs and NVMe

Josef Bacik
2019-12-22T17:43:33Z

On 12/21/19 1:24 AM, Chris Murphy wrote:
> Hi,
>
> Recent kernels, I think since 5.1 or 5.2, but tested today on 5.3.18,
> 5.4.5, 5.5.0rc2, takes quite a long time for `fstrim /` to complete,
> just over 1 minute.
>
> Filesystem Size Used Avail Use% Mounted on
> /dev/nvme0n1p7 178G 16G 161G 9% /
>
> fstrim stops on this for pretty much the entire time:
> ioctl(3, FITRIM, {start=0, len=0xffffffffffffffff, minlen=0}) = 0
>
> top shows the fstrim process itself isn't consuming much CPU, about
> 2-3%. Top five items in per top, not much more revealing.
>
> Samples: 220K of event 'cycles', 4000 Hz, Event count (approx.):
> 3463316966 lost: 0/0 drop: 0/0
> Overhead Shared Object Symbol
> 1.62% [kernel] [k] find_next_zero_bit
> 1.59% perf [.] 0x00000000002ae063
> 1.52% [kernel] [k] psi_task_change
> 1.41% [kernel] [k] update_blocked_averages
> 1.33% [unknown] [.] 0000000000000000
>
> On a different system, with older Samsung 840 SATA SSD, and a fresh
> Btrfs, I can't reproduce. It takes less than 1s. Not sure how to get
> more information.
>
>

You want to try Dennis's async discard stuff? That should fix these problems
for you, the patches are in Dave's tree. Thanks,

Josef